This invention relates generally to microwave switches and, more particularly, to microwave switches which use switching diodes.
As is known in the art, microwave switches generally include a microstrip circuit having an input or feed line and a plurality of output lines all lying in a single plane, such input line being coupled to the output line through discrete switching diodes connected between the input line and corresponding ones of the output lines. The switching diodes are generally two-junction semiconductor devices, sometimes referred to as P I N diodes. In order to obtain proper impedance matching between the input line and each of the output lines, it is necessary that the output lines be sufficiently separated from each other to prevent significant stray coupling or fringing effects. Further, to obtain the proper matching, it is necessary that the diodes have one electrode connected at substantially coincident points on the input line.
Because of the co-planar arrangement described above in many applications, as for example when greater than, say nine, output lines are required, the requisite separation and bonding criteria is impractical to achieve. Also, with such co-planar arrangement, because the input line is not symmetrically positioned with respect to the output lines, different ones of the output lines will have different degrees of stray coupling or fringing to the input line, and, hence, the impedance matching between the input line and each of the output lines will be correspondingly different. Further, the switching diodes are generally capable of handling only relatively low levels of power because, in the co-planar arrangement described above, a significant portion of the heat generated in the switching diodes passes through substantially the entire diode to the ground plane of the microstrip circuit. Still further, in such arrangement, because each of the switching diodes is a discrete device, the electrical characteristics may be different one from the other, thereby affecting the electrical switching characteristics of the input line to each of the output lines.